Replace hand-rolled MarkdownToHTML with goldmark

Replace the ~350-line regex pipeline (convertInline, placeholder
extraction/restoration, line-by-line parser) with goldmark configured
for Trix editor compatibility via three custom components:

- trixTransformer: forces tight lists, converts soft breaks to hard
  inside list items and blockquotes, inserts <br> between
  blank-line-separated blocks for Trix paragraph spacing
- trixRenderer: compact blockquote output, HTML escaping for raw
  HTML blocks (vs <!-- raw HTML omitted -->), TrixBreak and
  EscapedAt node rendering
- escapedAtParser: intercepts \@ before goldmark's standard escape
  handling to preserve mention-suppression syntax

HTMLToMarkdown gains <br>-aware round-trip support: blockquote and
list handlers now normalize <br> tags before splitting, so multiline
blockquotes and list continuations survive the edit loop
(MarkdownToHTML → HTMLToMarkdown) faithfully. Also fixes multiline
blockquote/paragraph regex matching ((?i) → (?is)) and trims
trailing newlines from code fence content.

Promotes goldmark from indirect to direct dependency (v1.7.13, already
present via glamour).

Author: jeremy

go.mod

@@ -19,6 +19,7 @@ require (
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/pflag v1.0.10
 	github.com/stretchr/testify v1.11.1
+	github.com/yuin/goldmark v1.7.13
 	github.com/zalando/go-keyring v0.2.8
 	golang.org/x/mod v0.34.0
 	golang.org/x/sys v0.42.0
@@ -70,7 +71,6 @@ require (
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e // indirect
-	github.com/yuin/goldmark v1.7.13 // indirect
 	github.com/yuin/goldmark-emoji v1.0.6 // indirect
 	golang.org/x/net v0.38.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect